Are there any application notes or design recommendations for daisy chaining CY7C65630 hubs? For example, connecting four CY7C65630 hubs downstream of a fifth at the board level, making 16 downstream ports available through a single upstream port.

There is no application note per se for this specific design but it should be just like laying out board for any other USB connection. Even though hub is connected downstream the upstream port of the hub is going to behave just like any other USB device.

Could I simply use the PWRn_b outputs of the upstream hub to pull the VBUSPOWER inputs of the downstream hubs high (using a buffer) only when the upstream hub has finished enumerating? This would then be similar to daisy-chaining discrete self- or bus-powered hubs, where the PWRn_b outputs of the upstream hub are used to enable VBUS on downstream ports via a power management chip.